1. Field of the Invention
This invention relates to a method of and an apparatus for adsorbingly fixing a body such as a mask or wafer, and more particularly to such a method and apparatus for eliminating any magnification error, with respect to the pattern of the preceding step, when a pattern of the photomask is transferred onto the semiconductor wafer in a semiconductor exposure apparatus used in the integrated circuit manufacturing process.
2. Description of the Prior Art
In the integrated circuit manufacturing process, there is a plurality of steps of projecting and transferring a mask pattern onto a semiconductor wafer. It is necessary that the mask pattern to be transferred in a subsequent step be highly accurately aligned over the entire surface of a wafer relative to the mask pattern transferred onto the wafer during the preceding step. This also holds true of the mask, but the case of the wafer is first considered. Generally, in an exposure apparatus, once a semiconductor wafer is vacuum-adsorbed to a wafer chuck, the vacuum is not released until exposure is terminated. However, it has been confirmed that if, in this condition, the wafer chuck creates a minute amount of expansion and contraction resulting from temperature changes, the wafer also is subject to an amount of expansion and contraction greater than the amount of expansion and contraction intrinsic to the wafer. For example, let it be assumed that the temperature around the chuck has risen by 0.2.degree. C. from when silicon wafer has been absorbed to the wafer chuck made of aluminum until exposure is started and that the temperatures of the wafer and the wafer chuck also have risen correspondingly.
Heretofore, in such a case, the amount of expansion of a 5-inch wafer has been only about 0.06 .mu.m due to the coefficient of linear expansion (2.5.times.10.sup.-6 /.degree.C.) of a silicon wafer and this has been considered to be a negligible amount. At the same time, however, the wafer chuck (coefficient of linear expansion: 23.times.10.sup.-6 /.degree.C.) would expand by about 0.6 .mu.m and so that the wafer would be pulled by the friction force between it and the chuck to exhibit an equivalent elongation, namely, an amount of expansion of approximately 0.6 .mu.m. Accordingly, when exposure is effected in such condition, a magnification error occurs between the pattern of the preceding step and the pattern of the current step and thus, highly accurate alignment of the wafer over the entire surface thereof becomes impossible.
In recent years, various methods for controlling the temperature of the wafer to thereby align the mask pattern and the wafer pattern highly accurately have been devised. However, if the temperature of the wafer chuck changes after the wafer has been vacuum-adsorbed to the wafer chuck, the wafer is pulled by the wafer chuck because the amount of expansion and contraction of the wafer chuck is great. Therefore, expansion and contraction exceeding the amount of expansion and contraction resulting from a temperature change of the wafer itself occurs to the wafer.
Conversely, in the case where the wafer is placed on the chuck after the temperature of the wafer chuck has been stabilized, if the wafer is vacuum-adsorbed before the temperature of the wafer becomes completely equal to the temperature of the wafer chuck, there occurs a phenomenon that expansion and contraction of the wafer is prevented by the friciton force between the wafer and the chuck and a desired amount of expansion and contraction cannot be obtained.
With such phenomenon, the amount of expansion and contraction of the wafer is determined by the fricton force between the wafer and the wafer chuck and the internal stress of the wafer being balanced, and the friction force between the wafer and the wafer chuck is affected by the state of the back surface of the wafer, the state of the front surface of the wafer chuck or the presence of dust between the wafer and the wafer chuck. Therefore, the amount of expansion and contraction of the wafer cannot reliably be reproduced and, even if the amount of expansion and contraction of the wafer chuck is taken into account, it has been impossible to effect the above-described highly accurate alignment compensation.
A similar phenomenon is considered to occur also in the case of a mask and a mask chuck.